1. Field of the Invention
The invention relates to internal combustion engines and, more particularly, to a method for detecting cyclical fluctuations in combustion in an internal combustion engine.
Operating internal combustion engines with lean mixtures and/or high exhaust gas recirculation rates offers a good opportunity to reduce fuel consumption and nitrogen oxide emissions. Extremely lean mixtures, of the kind that occur in lean engines with typical air numbers of around .lambda..apprxeq.1.4 or a high proportion of inert gases in the mixture as a result of the exhaust gas recirculation, however, have poorer flammability of the mixture as a consequence. That can lead to an increase in cyclical combustion fluctuations and even to combustion misfires, if the recirculation rates are too high or the mixtures are too lean.
For reliable operation of internal combustion engines at high recirculation rates or lean mixtures, it is therefore necessary to detect and weight the cyclical combustion fluctuations. Combustion in the cylinder of an internal combustion engine can be measured by measuring the cylinder pressure course with ensuing calculation of such characteristic variables as the indexed mean pressure. The cyclical fluctuations in these variables are directly proportional to the cyclical combustion fluctuations.
It has been known in the art to measure and evaluate the crankshaft speed in order to detect combustion misfires. In that process the period of time during which the crankshaft rotates by a certain angular extent, also called a segment, is measured. Combustion misfires lead to a temporary slowdown in the angular speed of the crankshaft and thus to an increase in the difference between successive periods of time.
U.S. Pat. No. 5,433,107 (EP 0 576 705 A1) describes a method for detecting combustion misfires via fluctuations in the angular speed of the crankshaft. There, the general trend of the engine speed and in addition non-uniform changes in the speed are taken into account. The method described there furnishes a nonconcentricity value LU.sub.n (referred to as "lack of smoothness value") which is proportional to the change in the angular speed of the crankshaft. The nonconcentricity value thus obtained is then compared with a limit value, and a combustion misfire is determined if the nonconcentricity value exceeds the limit value.